Unlocking Iron Redox Depth for High-Energy Layered Sodium Oxide Cathodes

High-capacity O3-type layered NiFeMn-based oxides are promising cathodes for sodium-ion batteries, though their practical deployment is constrained by the inherent limitations of Fe redox chemistry. Traditional designs generally enforcing stoichiometric symmetry (Ni ═ Mn) yield low Fe redox activity. Herein, we propose a valence engineering strategy that breaks conventional Ni/Mn stoichiometry to reconfigure Fe's local chemical environment and unlock unprecedented redox depth. Density functional theory (DFT) calculations reveal that the designed NaNi_0.35Fe_0.225Mn_0.425O₂ cathode exhibits a reduced Bader charge on Fe (1.598 vs. 1.638 in NaNi_1/3Fe_1/3Mn_1/3O₂) and elevated Fe 3d orbital energy, signifying enhanced Fe redox activity. This configuration enables an exceptional Fe^2.60+/Fe^3.88+ redox (1.28 e⁻ per Fe), delivering a reversible capacity of 184.3 mAh g⁻¹ within 2–4.2 V at 0.2 C, markedly exceeding the benchmark NaNi_1/3Fe_1/3Mn_1/3O₂ (161.3 mAh g⁻¹) with low reaction depth of Fe^3.01+/Fe^3.61+. The intensified cationic redox reaction enables an ultrahigh energy density of 596 Wh kg⁻¹. The NaNi_0.35Fe_0.225Mn_0.425O₂ cathode demonstrates robust performance over a broad temperature range from −15°C to 60°C. In situ and ex situ characterizations unveil a reversible O3 ↔ P3 ↔ OP2 phase transition with minimal volume change (1.88%) that circumvents detrimental deleterious O′3 intermediates and intragranular cracking. This work establishes valence engineering as a paradigm to consolidate cationic redox reaction in high-energy layered sodium oxide cathodes.